From FAO Irrigation and Drainage Paper 33 (Use the larger value of each pair if the area around the crop has no transpiring vegetation).

Table 2. Evapotranspiration Crop Coefficients for crops reaching 80-90% full ground cover by heading. Reduce the coefficients for crops with less ground cover*.

Growth stage

Zadoks code

Crop coefficient

Crop ground cover

Early vegetative growth

Z1.0 - Z1.3

0.3

10-30%

Tillering

Z1.3 - Z3.0

0.6-0.8

30-80%

Stem elongation to flowering

Z3.0 - Z6.8

1.05

70-100%

Grain filling

Z6.8 - Z8.7

0.40.4

50-20%

Step 2. Estimating available soil moisture

The crop begins to suffer stress and lose yield when around 50% of the available soil water has been used. Calculate the available soil moisture as follows:

Water available (mm)=Present soil moisture (mm)-Soil moisture (mm) when at 50% of field capacity.

Use the table to estimate soil moisture by feel. Estimate the current rooting depth of the crop.

Step 3. Estimating time until moisture stress

Now calculate the number of days that the crop can go without irrigation or significant rainfall before stress begins:

Number of days before stress=Available soil water (mm) (from Step 2)/Daily crop water use (mm) (from Step 1)

Example:

This calculation may seem fairly difficult so try an example to give yourself confidence.

Crop stage: Heading, good ground cover

Soil: Clay-loam soil (i.e., soil texture class)

Environment: Semiarid tropics, mean temperature 20° C.

Step 1: Estimating crop demand

Evapotranspiration = 7 mm water per day

(Table 1 above)

Crop coefficient = 1.0

(Table 2, column 3 above, crop is at heading)

Water demand = 7 mm * 1.0 = 7 mm water/day

(i.e., Step 1 calculation).

Step 2. Estimating available soil moistureThe soil is a clay loam that forms a ball when you squeeze it, but the ball breaks easily. According to the table to estimate soil moisture by feel your soil contains 110 mm water per m soil depth. Assuming the crop roots reach down to 1 m, then 110 mm water is available for crop use.

The table to estimate soil moisture by feel also shows that at 50% moisture, when stress begins, your soil holds 80 mm of available water, so the crop can use 110 - 80 = 30 mm before stress begins.

Step 3. Estimating time until moisture stressAt 7 mm per day water loss, you do not need to irrigate for about 4 days (i.e. 30/7 days).